I found FontForge which is a free and open source font editor. You can use it to make your own fonts. My favourite font for ASCII art is FixedSys, the one which comes with MS Notepad. It is thick/ bold so easy to view as I’m making something new or for capturing screenshots to share what I finish making.
I’m not quite ready to try making my own monospaced font. But, it would be an interesting project.